1. Field of the Invention
The present invention generally relates to dye laser resonators and to outcouplers; and more specifically, to non-confocal unstable dye laser resonators and a new type of outcoupler mirror device.
2. Description of Prior Art
Dye lasers form a very important class of lasers because they provide easily selectable wavelength laser radiation in the visible and near visible regions of the spectrum. The gain material of dye lasers has traditionally been liquid solutions of dyes. Dye doped plastics were tested but not found practical because of easy damage to the material. Recent advances have lead both to a better class of dyes as well as to damage resistant plastics so that dye doped plastics have now become practical laser gain materials. Some of the dye lasers--both liquid and solid state--have been demonstrated to be very efficient wavelength converters when pumped with a shorter wavelength laser such as the frequency doubled Nd:YAG laser at 532 nm wavelength. Conversion efficiencies as high as 85% have been demonstrated. In order for a laser to be most useful, energy efficiency is often not enough or even most important. What is often most important is the "brightness" of the laser which requires a low intrinsic divergence of the laser beam.
The typical dye laser resonator is plano-parallel where the dye gain material is placed between two parallel mirrors. One of these mirrors is fully reflecting at the dye laser wavelength and the other mirror (called "output coupler" or "outcoupler") is partly reflecting and partly transmitting. Such a resonator is highly efficient. The problem, however, is the poor beam divergence-diameter product which may be 100 mm-milliradians (or worse).
The confocal resonator, where the resonator length (d) equals the difference in the focal lengths of the two mirrors, is also commonly used. Confocal resonators are not compact and tend to be moderately long (&gt;&gt;10 cm) where all laser beams within and outside the laser cavity are collimated. Increased length means a smaller Fresnel number, N=a.sup.2 /ld, where a is the cavity gain aperture radius, and l is the lasing wavelength. The beam divergence is reduced in smaller Fresnel number cavities as the high order modes "diverge" out of the gain volume faster than the low order modes because of diffraction. In a confocal unstable resonator the instability upon each successive round trip occurs only in the size of the beam. The beam directionality, on the other hand, is stable, with the propagation vector being parallel to the resonator axis and the output wavefront is planar.
Mirror technology has also advanced in part due to laser design optimization of the prior art. Signal and energy pathways require movement of such pathways in one direction while reflection/redirection in another direction; a usual requirement in most laser design schemes. Partially reflective mirrors (one class of what is known in the art as: path altering elements) are especially effective in laser cavity resonators as they allow the characteristic oscillating optical field between mirrors, yet by virtue of the partially reflective mirror allows for a resultant emission out of that cavity mirror end. Gradient reflectivity mirror (GRM) unstable resonators have been developed to try to achieve a high brightness yield by manipulation of beam output pathways. The mirror that comprises one side of the cavity resonator is the GRM, which is designed as a partial reflective mirror having selected reflected surfaces thereon. It should be noted that whereas GRM's have been used with several other types of lasers, they have not been used with solid state dye lasers before. Further, resonators using a GRM are generally of confocal configuration (mirror separation equal to the difference in the focal lengths of the two mirrors) which renders them moderately long (&gt;&gt;10 cm). Decreasing the resonator length by bringing the mirrors closer together results in a highly diverging beam.
While the prior art has reported using dye lasers none have established a basis for a specific apparatus that is dedicated to the task of resolving the particular problem at hand. What is needed in this instance is a new non-confocal unstable laser resonator resulting in compactness and high brightness.